The present invention relates to electrographic printers of the type wherein a printhead array generates charge carriers and directs them at a recording or imaging member by the selective activation of crossed, but not necessarily orthogonal, row and column electrodes. It is particularly directed to such printers wherein one set of electrodes is activated with a voltage to function as a source of charge carriers, and a second set of electrodes are activated to extract and accelerate the charge carriers toward a latent imaging member.
Printheads of this type are described in U.S. Pat. Nos. 4,160,257, 4,628,227, and others. In the printheads described more particularly in the aforesaid patents, a set of electrodes are activated with an RF frequency signal of several thousand volts amplitude to create a localized corona or glow discharge region. Lesser control voltages are applied to one or more control electrodes located at or near the discharge region in a manner synchronized with the RF actuation to gate positive or negative charge carriers from the region, and the printhead is biased with respect to a dielectric member to maintain an accelerating field therebetween, thus depositing latent image charge dots on a dielectric imaging member as it moves past the printhead.
In printing devices using this type of printhead, the RF driven corona generation lines extend along the width of the printhead, spanning many of the control electrodes, which cross them at an angle. One commercial embodiment, by way of example, has twenty parallel RF lines, which are crossed by one hundred twenty eight oblique control electrodes, known as finger electrodes. During the time when one RF line is activated, by a burst of approximately five to ten cycles of a one to three MHz drive signal with a peak to peak amplitude of approximately 2700 volts, those finger electrodes which cross the RF line at the desired dot locations are activated to deposit charge dots.
In the conventional drive circuitry for such systems, the RF drive lines are actuated in a fixed sequence independently of the image being printed, while during any given RF line actuation, the number of finger electrodes which are actuated varies in accordance with the pattern being printed. After a slight delay for the RF voltage to ramp up, the designated finger electrodes are turned on to cause charge carriers to pass from the printhead and accelerate toward the drum, belt or other latent imaging member. Specifically, during their "OFF" cycle, each finger is back biased by several hundred volts with respect to the screen voltage; during their "ON" cycle, the finger voltage is switched to approximately the same potential as the screen.
In the original printers of this type, the finger electrodes were switched on for a fixed interval substantially co-extensive with the RF corona generation burst. Such operation produces a fixed amount of charge per actuation. More recently, in U.S. Pat. No. 4,841,313 of Nathan K. Weiner, constructions with a finger pulse of varying duration have been proposed. This operation varies the amount of charge deposited at each dot.
In printheads of the aforesaid type, the positive or negative half-cycles of individual RF waves applied to the RF electrodes generate charge carriers, and thus define a basic quantum of charge which may be deposited as a latent image dot. In order to achieve a reasonable range of gray scale charge values using the control of U.S. Pat. No. 4,841,313 it is therefore necessary to provide a larger number of cycles in each burst of the RF line drive signal. This requires the use of a higher RF frequency, or a greater interval of time, for printing each dot, thus entailing trade-offs either in terms of circuit cost or of machine operating speed.
Accordingly, it is desirable to print gray scale charge levels from a printhead array without increasing the number of cycles in each RF burst or its duration.
It is also desirable to operate a printhead array to achieve gray scale charge levels using a small number of simple control signals.